1. Field of The Invention
This invention relates to organopolysiloxane compositions and more particularly, to ultraviolet light-curable organopolysiloxane compositions which are readily curable by irradiation of UV light at room temperature within a short time and are useful for release paper.
2. Description of The Prior Art
It is known that organopolysiloxane compositions are applied onto an appropriate substrate and irradiated with UV light to cure the coating, thereby forming a thin film of the silicone cured product. The organopolysiloxane composition used for this purpose has been proposed, for example, in Japanese Laid-open Patent Application No. 56-38350. The composition set forth in this application is comprised of an organopolysiloxane having an epoxy functional group in the molecule and an onium salt photoinitiator.
The onium salt initiator is sparingly dissolved in the epoxy group-containing organopolysiloxane. When the initiator and the organopolysiloxane are mixed together, the resultant mixture is not uniform wherein the photoinitiator is readily separated or settled down. If such a mixture is applied onto a substrate, the coated film becomes opaque. In this condition, if UV light is applied, uniform transmission through the film is not possible, resulting in a non-uniform degree of curing of the film.
To avoid this, attempts have been made to improve the miscibility of the epoxy group-containing organopolysiloxane and the onium salt photoinitiator. In such attempts, there have been proposed in Japanese Laid-open Patent Application Nos. 64-186 and 64-29486 polyorganosiloxane compositions wherein functional hydroxyl group-bearing organic group, or polyether-polymethylene groups are introduced into the epoxy group-bearing organopolysiloxane. In order to prepare such polyorganosiloxanes, it is necessary to prepare the epoxy group-bearing organopolysiloxane through addition reaction between epoxy compounds having an ethylenically unsaturated bond and an organohydrogenpolysiloxane while subjecting simultaneously to hydrosilylation reaction of a hydroxyl group-containing compound having an ethylenically unsaturated bond or a polyether compound having an ethylenically unsaturated bond. This is not advantageous in that the reaction does not proceed in an efficient manner unless it is carried out under severely controlled conditions, with a low yield of intended product.
For improving the miscibility of the epoxy group-bearing organopolysiloxane and the onium salt photoinitiator, there has also been proposed modification of the organopolysiloxane with carboxylic acids such as benzoic acid (Japanese Laid-open Patent Application No. 1-297421). In the case, removal of the unreacted carboxylic acid after modification therewith is difficult. If the acid is left, the polymerization of the polysiloxane through the epoxy group will be induced, thus presenting a problem on the storage stability. In addition, another problem will be involved in that the cured film obtained from the modified epoxy group-bearing organopolysiloxane is more liable to undergo tight release than that obtained from non-modified organopolysiloxane.